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®PerdidoWorld’s deePesT oFFsHore drillingand ProducTion PlaTFormsupplement to66COMPANY PROFILEtwo extremes define Perdido’s early develop-ment wells: the water is very deep, and thereservoirs are very shallow below the mudline.the difficulty for drillers is the small differencebetween the amount of pressure required tocontrol the wellbore and the amount of pressurerequired to fracture the rock around it.In the silvertip field, where schlumbergercompleted Perdido’s first two developmentwells, the narrow pressure window is so lowthat conventional open-hole gravel pack treat-ments would be at risk.“the challenge is to pump the gravel slurryat a low enough pressure to keep it fromfracturing and entering the formation, withoutreducing the effectiveness of the gravel place-ment,” says salah Al-harthy, schlumbergerwell completion engineer. “the efficiency of thegravel pack directly affects the productive life ofan open hole well. Any voids will expose thescreens to erosion and possible well failure.”Perdido’s first two development wells weredrilled from a floating rig in the silvertip field,in 9,400 feet of water. the targeted zonewas the Oligocene-Frio sands, a portion of theLower tertiary that consists of as many as sixseparate reservoirs from 1,500 to 2,500 feetbelow the mudline.the wells reached a maximum total depthof 16,513 feet, with horizontal open-holelength of 2,300 feet. since the Oligocene-Friosands may compact as they are produced,well planners decided to complete thesesands with a horizontal open-hole gravel packto enhance the longevity of these completionsin a highly compressive reservoir.“there were two primary challenges incompleting these sands with horizontal open-hole gravel packs,” Al-harthy says. “the firstchallenge was the narrow pressure windowand the second challenge was the selection ofcarrier fluid.”Designing the completionsthe fracture gradient of the targeted Oligo-cene-Frio sands was about 7,100 psi. withreservoir pressures of 6,700 psi, that meantthe pressure window for placing the gravelpack was only 400 psi. the risk of such anarrow operating window is that the reservoirrock can fracture during treatment, causingpremature screen-out and insufficient packingaround the screens.the requirement for the gravel packingcarrier fluid needed to have enough carryingcapacity to suspend the gravel and removethe filter cake after the gravel was placed.It also needed to have a low corrosion ratearound the sand control screens, since thewells were being completed more than ayear before first oil.to overcome these challenges, schlum-berger used two of its flagship technologies,the OptiPac* Alternate Path†systems, and itsClearPAC* Ms gravel packing fluid system,to ensure a complete pack around the sandcontrol screens.OptiPac Alternate Path systems use shunt-tube technology to fill the voids that can formduring gravel packing even under fracturingconditions. the shunts are installed along thescreens, each with a series of evenly placednozzles.“when a bridge forms, the slurry naturallydiverts into the shunts,” Al-harthy says. “whenslurry encounters a nozzle facing a void inthe open hole, it flows out to fill the unpackedannulus, and the process continues until theinterval is fully packed.”sChLUMBERgER PIONEERs RECORD DEEPwAtEROPEN-hOLE gRAVEL PACK COMPLEtIONsHorizontal open hole completion with OptiPacOptiPac screens and cutaway sectionof a nozzleOptiPac systems are part of an inte-grated approach that ensures that screensare fully protected in high flow-rate wells, andis utilized in-conjunction with sandCADE*,gravel-pack design and evaluation software tohelp engineers design, execute and evaluatethe entire job.ClearPAC Ms, which is an advancedviscoelastic surfactant, was selected as thecarrier fluid on the first two silvertip wellsbecause of its excellent sand suspensioncapabilities, even at very low shear rates.the sand suspension and low friction pressurecharacteristics allow it to perform exceptionallywell in the long intervals of both open holeand cased hole completions. the ClearPACMs fluid also incorporates a breaker system(based on MudsOLV technology) to helpdissolve filter cake that remains in the wellboreafter the gravel pack is completed. By combin-ing the breaker system with the carrier fluid thisallowed shell to eliminate the need to reenterthe well, saving valuable rig time.“to compensate for the narrow operatingpressure, we also modified the mixing equip-ment on our stimulation vessel,” Al-harthy says.this equipment, called the POD* pro-grammable optimum density blender, mixesthe fluid and the gravel before it enters thepump. the POD normally operates at ratesof above six barrels a minute, but for thePerdido wells the equipment was modifiedto accept half the rate of fluid and proppant,to maintain pumping pressures within thenarrow margins.“we conducted a number of trials andextensive modeling beforehand to make surethat the equipment, fluid and overall designwould perform as planned,” says Al-harthy.Monitoring in real timethroughout the process, shell and schlum-berger engineers in distant locations wereable to collaborate efficiently in real timeusing the InterACt* connectivity, collabora-tion, and information system. InterACt’s robustdesign allows it to continue working, even inareas with marginal communication links.Working the plan“we really did our homework,” Al-harthyadds. “Using powerful models andsimulations, we worked together in a highlycollaborative environment with shell, toevaluate all the risks and to ensure successfulexecution. Perdido is a frontier development,so there were many uncertainties, but at theend of the day the modeling was correctand these jobs were performed as designed.the work we did on those first two wellswill guide many of Perdido’s future wellcompletions.”*Mark of schlumberger†Mark of ExxonMobil Corp.; technology licensedexclusively to schlumbergerSchlumberger1325 s. Dairy Ashfordhouston, tX 77077U.s.A.tel: +1 281 285 1300website: www.slb.com/sandcontrolClearPAC MS Fluids are used in OptiPac operationsto ensure proper gravel transportation and controlledleakoff through the nozzles of packing shunts.MudSOLV: Filter Cake BreakerPart A shows nonaggressive, slow-acting, MudSOLVsolutions remove the filtercake more uniformly,resulting in higher retained permeability across theentire interval.Part B shows aggressive treatments like acid createlocalized open areas (wormholes or pinholes) in thefiltercake, leaving large areas of filtercake unremoved,generally over areas having lower permeability.Schlumberger DeepSTIM II stimulation vessel